This page describes longitudinal data sets, some of them publically available, some for participating investigators.
ADNI
The Alzheimer's Disease Neuroimaging Initiative
Three stages (ADNI, ADNI-GO and ADNI2) exists with 800-1000 subjects and several time points approx. 6 months to a year apart.
See also the world wide ADNI:
http://www.alz.org/research/funding/partnerships/ww-adni_overview.asp
OASIS
Longitudinal MRI Data in Nondemented and Demented Older Adults
This set consists of a longitudinal collection of 150 subjects aged 60 to 96. Each subject was scanned on two or more visits, separated by at least one year for a total of 373 imaging sessions
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2895005/
MIRIAD
Minimal Interval Resonance Imaging in Alzheimer's Disease
The MIRIAD dataset is a database of volumetric MRI brain-scans of 46 Alzheimer's sufferers and 23 healthy elderly people. Many scans were collected of each participant at intervals from 2 weeks to 2 years and includes 708 scans.
http://www.ucl.ac.uk/drc/research/miriad/
http://www.ncbi.nlm.nih.gov/pubmed/23274184
PPMI
Parkinson's Progression Markers Initiative
ELUDE
Efficient Longitudinal Upload of Depression in the Elderly
The ELUDE dataset is a longitudinal study of late-life depression at Duke University. There are 281 depressed subjects and 154 controls included. An MR scan of each subject was obtained every 2 years for up to 8 years (total of 1093 scans).
DIAN
The Dominantly Inherited Alzheimer Network
http://www.dian-info.org/resourcedb/
ABIDE
Autism Brain Imaging Data Exchange
http://fcon_1000.projects.nitrc.org/indi/abide/
FITBIR
Federal Interagency Traumatic Brain Injury Research
FITBIR may have some longitudinal data (after obtaining an account).
https://fitbir.nih.gov/jsp/access/index.jsp
CCBD One-month Test-Retest Reliability and Dynamical Resting-State Study
30 subjects scanned 10 times within a month (anatomical, resting state, DTI, ASL). Center for Cognition and Brain Disorders (CCBD) at Hangzhou Normal University (HNU) http://fcon_1000.projects.nitrc.org/indi/CoRR/html/hnu_1.html